The Myth of Sufficiency

A widespread myth of sensor resolution (or the end of the megapixel race) states that if you print larger, you will step back further and, consequently, will never need the resolution given by higher megapixel cameras and prints. This myopic view of megapixels is correct if you are thinking of snapshots, looking solely at web uploads, or preventing* people from getting close enough to inspect the images.

* Helmut Newton’s big nudes are hung above the stairs in the lobby of the Berlin Museum, where it is physically impossible to get closer than about 2 meters, to realize that they are really not that sharp.

Bryce canyon at dawn, Utah, USA

Bryce canyon at dawn (seen from sunset point). One of my best-printing images. Linhof Technika 4×5, Fujichrome Provia 100. Scanned by MSP Graphic Services, Cotati, CA, on a Dainippon Screen drum scanner. 160 MP, corresponding to 15.7 x 19.6″ at 720 dpi, 16 bit, 920 MB tiff. Down-sampled to 2200 pixels on the narrow side and badly jpeg compressed to arrive at a 1.4 MB file size for web upload.

The images in my web gallery are heavily compressed jpegs of about 5 MP. An Apple Thunderbolt Display runs at 109 ppi  (2560 x 1440) while the highest resolution display is the one of the iPad Mini retina at 326 ppi (2048 x 1536). My images therefore qualify to be viewed at full size on the iPad and the Thunderbolt displays. As long as you can still focus that close, you can stick your nose to the iPad’s screen without beeing able to distinguish single pixels. But the images look less than optimal on the Thunderbolt when viewed from less than about a foot (30 cm)  away. There is simply no way a screen can do an image justice. With no universal standard for monitor calibration, it is difficult to ensure that you will see on your monitor what I did on mine.

Bryce-100

Crop from the above (click to enlarge). On the pixel level, film really isn’t all that sharp. But at 720 dpi there are still 4 times more pixels to make up for tonal separation. Bear in mind also, that displaying this image on a Thunderbolt corresponds to viewing a 100″ (2.5 m) wide print from a one foot (30 cm) distance.

On the same footing that I warn my friends about photographing their kids exclusively using the iPhone, I should review my own work. For more than 20 years I had my work printed on Cibachrome, because there was simply no other way of displaying the large-format images, if we forget the loupe and light table.  And pre-viewing b&w and color reversal film was impossible altogether. I have to admit, however, that pixel-peeping has more and more replaced this habit after the advent of film scanning and the all-digital workflow. But some recent visits to galleries made me think: I saw scanned film made to look like collodion wet-plate, film made to look digital, digital made to look like Cibachrome, and sub-inch sensor digital blown up to gallery-size. And all that with limited success; mediocre printing techniques as an artistic statement, a sign of carelessness, or simply bad craftsmanship?

Making a print is the final part of the photography workflow. Without a print, there is no reference on how an image should be viewed and what quality of capture is required. I would define a high-impact, technically perfect print as evoking the following reaction among the viewers:  they stop at a distance where they can see the whole image, say 2 meters for a 29” (740 mm) print. Realizing the image is sharp and detailed, they move in closer and closer as long as they can still focus. This is what I call supernatural, because the print reveals details in extend of the visual acuity at the location. So the question is: what is the point of sufficiency, or how much is enough?

Visual acuity, a measure for distinguishing details, is defined as the reciprocal of the angular distance that must separate two contours so that they appear as discrete; a = 1/r, where a is the (dimensionless) acuity and r the response, expressed arc-minutes per line pair. The acuity is 1.1-1.7 under good lighting conditions*. The acuity of 1.5 corresponds to 0.66 arc minutes per line pair. For a minimum of two pixels per line pair, we have a pixel spacing of 0.33 arc-minutes, that is, 0.33*Pi/10800 radians = 0.1 mrad or 0.1 mm (an eyelash) at one meter distance.

*S. Hecht: The Relation between Visual Acuity and Illumination, The Journal of General Physiology, 1927

Consider now viewing an A1 print (590 x 740 mm, 23 x 29″) from 29” distance. The wide side of the print subtends an angle of 0.9 radian, or 53 deg, which roughly corresponds to the picture angle of my 45 mm PC-E lens. At least 10000 pixels are therefore required to show detail at the limits of the human visual acuity. This corresponds to a printer resolution of 345 ppi, which is roughly that of the iPad Mini retina, albeit at a 37” diagonal instead of the 7.9” of the iPad Mini.

For the A1 print, a minimum of 80 MP is therefore required, the Nikon D800E will do for an A2 print, and an APS-C sized sensor for an A3. So size does matter. However, a 64” inch (1.6 m) print viewed at 64” distance still requires 10000 pixels on its long side. This is good, because we do not need more resolution as long as the viewing angle remains constant. But if we choose to examine only a small portion of this print we would be disappointed to see pixelation, print dots, and smudging. And this also means that on subjects with semi-fractal detail, such as forests and desert landscapes, we can see a difference between 16 and 36 MP even at print sizes as small as A4.

That’s just pure theory, one might say. So let us examine what all this means in practice. For the sake of scientific reasoning and for not wasting too much ink and paper, here is a proposal of a test sequence, up for discussion:

  1. Check how print quality can be discussed and judged objectively, in particular on an Internet forum. I am thinking of high-resolution scanning or macro-reprography of the finished print.
  2. Compare inkjet (Giclee) prints to C-prints (sometimes called Lambda or LightJet prints) and the good old Cibachromes. What defines the organic look (or the digital look) often alluded to in film versus digital debates.
  3. Determine the effective resolution of the printer, both with and without external RIP (raster image processor) software. Use test charts for resolution and real-world images to check a loss of tonal separation and micro-contrast. Note that inkjet printers use a dithering process to make up intermediate colors and tones. This requires a much higher dpi resolution of the printer compared to the ppi resolution of the file. On the other hand, imperfectly overlapping ink dots may even help to create the impression of subpixel detail rendering.
  4. Test the requirements on the paper. It must have a very fine fiber structure to not limit resolution, but also a high enough density to take a large quantity of ink.
  5. Compare a stitch of 4-6 (possibly not tack sharp) images to an upsized, perfectly sharp image captured at 36 MP**. Study the difference in apparent image quality of the final print depending on the mathematical model used for uprezzing, i.e., BiCubic, Lanchoz, Spline, etc. Is uprezzing best done in Photoshop or in the RAW developer?
  6. In case of film scanning, compare a scan at the resolution limit of the slide film (about 2400 dpi) to a largely oversampled scan on a high-end drum scanner. Is it worth to spend 150 $ (US) for a high-end drum scan or are the Flextight scans sufficient for a 29” print?
  7. Define criteria for over sharpening other than the obvious halos around fine detailed structures. Can added noise in Photoshop help to make the print look more organic? Conversely, does (even minimal) noise reduction result in smudging?

Lots of work ahead. SR

** A similar sized print from 8×10″ looks richer than one from 4×5″, which in turn looks better than a print from medium format,  although the micro-level acuity goes the other way. But this could mean that a 60+ MP full frame sensor (equivalent of 4 Olympus OM-D Micro-four-thirds) yields better prints than the 36 MP sensor of the D800E in spite of poorer pixel acuity, increased noise, and limiting lens performance. Why is this not produced? I guess, the camera designers feel obliged to satisfy the pixel peepers, and therefore we find this aggressive, in-camera sharpening. As my review of the Leaf Credo has shown, the manufacturers of the medium-format backs, serving the professional market, seem to be less stressed in this regard.

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6 Comments

  1. Jorge July 26, 2014 at 10:32 #

    Stephan, of course will be little differences in print. So what? I’m not against the pursuit of technical perfection in photography. What I’m trying to say is that it won’t turn a bad photograph into a good one, nor a good one into something extraordinary. In my opinion, it’s something accessory.

    How you look, how you choose to compose, what you put in the frame, how you play with light; these are the important aspects of photography. And you can do it being the owner of a Linhof Master Technika or a P&S camera. Technically, of course, the results won’t be the same; but if you don’t have an artistic gift there’s no gear in the world to get it. You’ll be printing 46 x 58” technically perfect photos that don’t say absolutely nothing.

    Everyone can shoot a camera, but only a few can call themselves photographers. To feel the passion is not enough. And, on the other hand, I don’t believe the best fine-art photographers think that much in their equipment. I reckon they concentrate in what I said before.

    • Stephan July 26, 2014 at 11:02 #

      I agree, of course, with what you say about subject, light, and composition. But discussing what makes an outstanding, artistic image is no easy task. I have tried this with my comments on kitsch in photography, which may trigger controversial discussions, however.

      I tend to concentrate on technical matters in my blog, which also reflects my educational background as engineer/scientist. If you bother doing something at all, you might as well do it to the best of your ability. This is why I am never fully happy with the ultimate image quality achievable in large prints.

      There is another aspect about equipment that I take for granted: knowing the limits. No need to buy an Otus for stopping down below the diffraction limit, using the Linhof in a windy environment or not getting the shot at all because all this equipment sits in the hotel room.

  2. Jorge July 25, 2014 at 19:24 #

    Great photographs don’t have anything to do, fortunately, with technical aspects. These are left for people without any imagination. Some iPod shots are artistically better than the whole production of many medium format pundits. At the end of the day, it’s people who makes a photo, not a machine. Andreas Feininger said it right: “Photographers — idiots, of which there are so many — say, “Oh, if only I had a Nikon or a Leica, I could make great photographs.” That’s the dumbest thing I ever heard in my life. It’s nothing but a matter of seeing, thinking, and interest. That’s what makes a good photograph.”

    As the Chinese proverb goes, “when a wise man points the moon the fool looks at the finger.”

    • Stephan July 25, 2014 at 22:05 #

      Jorge, I agree that iPod shots may be art while commercial work, using medium format, may be utterly kitsch. But this is a bit like saying: health is better than wealth. Look at the work by Vittorio Sella at the turn of the century; the 19th to 20th, that is. He captured the most remote places (even at today’s standards) in beautifully composed and technically perfect images. And why should we not strive to achieve such technical standard with more than 100 years of camera development to boot?

      In my opinion, art has to do with artistry in the sense of skill and craftsmanship. Andreas Feininger, among many others, elaborated his darkroom techniques and did not bring his film to One Hour Photo.

      My point is about the myth that the equipment has become so advanced, that it will be impossible to see any difference in print.

  3. Mike May 21, 2014 at 10:20 #

    Establishing criteria for perceived image quality is no easy task. Most analyses that appear in Internet discussions concern themselves primarily and often exclusively with what is called, reductively, “resolution.” More sophisticated articles discuss Modular Transfer Function (MTF), which at least can specify contrast of an image of alternating black and white bars at a given spatial frequency. This can be a good way to quantify what is perceived as “sharpness” for an image viewed at a certain distance. And that is where most of these discussions end. In reality, our “quality detector” is far more complex. At a minimum, a critical observer is aware of MTF, or micro-contrast of fine detail (down to 15 lines/mm at normal reading distance), density of tonal and hue variation, freedom from artifacts such as haloing and fringing, color distortion, and banding, posterization, and noise, and smoothness of gradations. Some of these objectives are in tension: Unsharp masking can enhance perceived sharpness of details in a certain range of size while creating gradation-killing haloing and distortion of even finer detail. Dithering can be introduced to increase smoothness that simultaneously flattens color nuances. Grain can be suppressed at the expense of fine detail, and so on. In general, the smaller the capture format and consequently the greater the enlargement for a given print size, the more visible are the compromises made to obtain sufficient image quality. The taste of the camera designers (in the case of digital cameras) and the photographer/printer also comes into play. And to some extent camera design influences public tastes, as perceptions of image quality tend to scale with the perceived limits of available quality. There is no question, for example, that “punchy,” colorful images are today valued more highly than in the past, as this is a design characteristic of most consumer-level digital cameras, as well as the now-popular plasma television displays.

    Quality comparisons are further complicated by the attempt to compare not just image captures of different sizes but of different type. If one must do this it would be best to compare systems with the same capture area, for example 35mm film cameras and 35mm DSLRs. Only by isolating the thing to be examined can we come to any sort of clear idea of it. Capture size has such a dominant effect on perceived image quality that it must examined separately from the choice of capture technology. Likewise, lens optics should be separated and considered by itself. Once these factors are considered in isolation one can begin to address questions such as the choice between a high-quality 35mm DSLR and a medium-format digital camera.

    A word about film scanning: Most scanners use capture technology quite similar to that employed by digital cameras and suffer from the same defects. This “observer effect” badly skews the comparison of film and digital captures. The only type of film scan that can faithfully record the finest detail that film can record and doesn’t distort tonalities is a photomultiplier-tube drum scan. These devices have extremely small sampling apertures (down to 1/12,000″) and apochromatic microscope objectives of large relative aperture (and thus great freedom from diffraction effects) as well as absence of the flare that afflicts CCD-type scanners.

    I must politely correct you on the limits of film resolution, which you place at 2,500 dots per inch. In fact, a well-focused image on fine-grained film can reveal detail down to 6,000 dpi or better. This might be visible in very great enlargements if the scanner or enlarging lens can resolve it. This is in fact the case with PMT drum scanners and excellent enlarging lenses such as the Apo-Rodagon

    • Stephan May 21, 2014 at 13:08 #

      Thanks Mike for this very comprehensive comment. I would appreciate you provide a guest posts on scanning and/or printing. And thanks for reminding me: for the film versus digital comparison, I must have my 4 x 5″ transparency scanned on a drum scanner.

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